The present invention relates to an electrochemical capacitor that exhibits a high electrostatic capacity density based on pseudo-capacity by using a carbon material having fine crystals of graphite-like carbon as a polarized electrode material, and by using a reaction product of an electrochemical reaction as an active material.
An electrochemical capacitor (hereafter called xe2x80x9ccapacitorxe2x80x9d) is used in a backup power source for electronic devices and in batteries for various transport machines such as motor vehicles because of its large farad-class capacity and excellent charge-discharge cycle characteristics. The use in the storage of electric power at nighttime has also been examined from the viewpoint of effective use of energy.
A single electrode cell 10, one of the basic structures of such a capacitor, generally comprises as FIG. 1 shows, a positive-side polarized electrode 24 and a negative-side polarized electrode 26 formed on current collectors 20 and 22 consisting of a metal material, respectively. The polarized electrodes 24 and 26 are separated by a separator 28, and are impregnated with an electrolyte solution consisting of a solvent and an electrolyte.
FIG. 2 shows the structure of a single capacitor cell 12, in which a plurality of single electrode cells 10 are electrically connected in parallel by electrode extensions 30 and 32 formed on current collectors 20 and 22. Such a single capacitor cell 12 is suitably used in a capacitor of a relatively large capacity used for motor vehicles and the like. These single electrode cells 10 and single capacitor cell 12 are both flat plate types, and are characterized in the ease of increasing packing densities and surface areas.
In addition to such a flat-plate type capacitor, there is a winding-type capacitor 70 as FIG. 3 shows. In a winding-type capacitor 70, a wound body 76 fabricated by cylindrically winding a positive electrode sheet 72 comprising a positive-side polarized electrode 24 formed on a current collector 20 and a negative electrode sheet 74 comprising a negative-side polarized electrode 26 formed on a current collector 22 via a separator 28 is used as an electrode body. For example, this wound body 76 is accommodated in a case 78 filled with an electrolyte solution, and the open end of the case 78 is sealed with a seal plate 82 on which electrode terminals 80 are formed, while securing electrical conduction between the electrode sheets 72, 74 and the electrode terminals 80.
As a material of the polarized electrodes for such capacitors, an activated-charcoal-based material that has a specific surface area of 1000 m2/g or more has been used. The major electrostatic capacity produced in such capacitors using activated charcoal is electric double layer capacity. In this case, the electrostatic capacity density is limited to 20F/cc, and the withstand voltage is limited to about 3 V.
In order to further improve the performance of capacitors, a new material for polarized electrodes that has a high electrostatic capacity is demanded, and a capacitor utilizing pseudo-capacity by a redox reaction of a metal oxide such as ruthenium oxide or a conductive high polymers such as polypyrrole is attracting attention.
However, since capacitors that use metal oxides require expensive materials such as ruthenium, and can use only aqueous electrolyte solutions, the operating voltage cannot be elevated. When a conductive high polymer is used, although the withstand voltage can be elevated because an organic electrolyte solution can be used, chemical synthesis is complicated and expensive, and thick films are difficult to form, making an increase in the capacity difficult.
Considering the above problems, the object of the present invention is to provide a capacitor that exhibits a high electrostatic capacity based on pseudo-capacity, and uses an active substance that can be synthesized at low costs.
According to the present invention, an electrochemical capacitor is provided, comprising an organic electrolyte solution, and polarized electrodes immersed in the organic electrolyte solution, wherein a reaction product formed by an electrochemical reaction of a carbon material that has partially oxidized fine crystals of graphite-like carbon in the organic electrolyte solution as an active substance of the polarized electrodes.
In the electrochemical capacitor of the present invention, the carbon material is preferably an easily graphitizable carbon material that has been heat-treated in an inert gas atmosphere at a temperature between 500xc2x0 C. and 1000xc2x0 C. One of the methods of partial oxidation comprises a heat treatment of the carbon material In the presence of at least one of an alkali metal compound that contains an alkali metal and oxygen in an inert gas atmosphere above the temperature at which a vapor of the alkali metal is generated. Another method of partial oxidation comprises the immersion of the carbon material in an oxidant that forms graphite acid when graphite is immersed therein. Furthermore, heat treatment of the carbon material may be carried out in an oxidizing atmosphere that contains an oxidizing gas at a temperature below the carbonization temperature of the easily graphitizable carbon material.
The electrochemical reaction for generating electrostatic capacity is preferably carried out by immersing an assembly of a sheet-like electrode fabricated using the partially oxidized carbon material, a separator, and a current collector in an organic electrolyte solution to construct a capacitor, and then passing an electric current through the capacitor. In this case, the electrolyte solution used for carrying out the electrochemical reaction may be different from the electrolyte solution that acts as a capacitor.